It will depend a lot on what the cost of electricity is in your area, would you use a loop field or a pump and dump, how much heating vs. cooling will you need to do, how well insulated is your house, etc.

I have a geo system w/ a pump and dump. Put it in 9 yrs ago. At the time it looked as if NG prices were going to go through the roof. Since then, with the advent of fracking, the supply of NG has increased thus lowering the price. If my geo were to crap out on me now, I would replace w/ a NG boiler and forced air unit since my heating days significantly exceed my cooling days.

You really should talk with a reputable HVAC contractor as there are a lot of factors that go into which system if the most efficient.

You may also want to look at an air source heat pump. That may make more sense, especially where you're located.